Measurement devices for measuring deflections of the upper vehicle seat part relative to the lower vehicle seat part and comprising a seatbelt-retracting mechanism are known in the art. One end of said seatbelt-retracting mechanism is connected to the upper vehicle seat part, one end of the seatbelt being connected to the lower vehicle seat part. Some of the seatbelt is therefore pulled out of the seatbelt-retracting mechanism and arranged between the upper vehicle seat part and the lower vehicle seat part. When the upper vehicle seat part is deflected relative to the lower vehicle seat part, the seatbelt is pulled further out of the seatbelt-retracting mechanism or is drawn further back into the seatbelt-retracting mechanism, depending on the direction of the deflection experienced. The deflection of the upper vehicle seat part can be determined on the basis of the change in the diameter of the seatbelt rolled up inside the seatbelt-retracting mechanism.
This type of measurement is, however, quite inaccurate since the diameter of the rolled-up seatbelt can never be determined exactly due to the resilience inherent in the seatbelt material the seatbelt consists of. It is not possible for the rolled-up seatbelt to have a constant diameter either, which can further distort the measurement result. Furthermore, this method involves an indirect measurement that can result in further measurement result errors.